1. Field of the Invention
This invention relates to a bicycle wheel rim, more particularly to a bicycle wheel rim which has an enhanced strength to resist impact applied on tire retaining walls thereof.
2. Description of the Related Art
Referring to FIG. 1, after a bicycle is in use for a period of time, the drive chain 1 thereof might become loose and slightly lengthen. At this time, the drive chain 1 can be easily released from the chain wheel 31 to result in inconvenience during use of the bicycle. As such, an adjustment device 3 has been provided to permit adjustment of the position of the axle 20 of the rear wheel on the bicycle frame. The adjustment device 3 is mounted on the chain stay 32 and the seat stay 30 of the bicycle frame, and includes a plate member formed with an elongated slot 300 for retaining the axle 20 of the rear wheel therein. When the drive chain 1 becomes loose, the axle 20 can be moved along the slot 300 to permit the drive chain 1 to engage fittingly the chain wheel 31. However, since the rear brake 4 of the bicycle is mounted on the seat stay 30, and since the wheel rim 2 of the rear wheel moves together with the axle 20 during the adjustment operation, unless a corresponding position adjustment is made for the rear brake 4, the brake pads 40 (only one is shown) of the rear brake 4 cannot be kept in proper frictional contact with the brake pad contacting surfaces formed on the tire retaining walls 21 of the wheel rim 2 after the axle 20 has been moved to an adjusted position (shown in phantom lines in FIG. 1).
FIG. 2 is a sectional view of the conventional wheel rim 2, which is shown to include left and right tire retaining walls 21 having brake pad contacting surfaces 21a, an inner spoke mounting wall 23 interconnecting inner edges of the tire retaining walls 21 and formed with a set of spoke fastening holes 26, and an outer connecting wall 22 surrounding the spoke mounting wall 23 and interconnecting the tire retaining walls 21. Although the connecting wall 22 imparts strength to the wheel rim 2, the strength of the wheel rim 2 is still insufficient to resist impact applied on the tire retaining walls 21. In addition, the spoke mounting wall 23 buckles easily due to the tension of the spokes (not shown) mounted thereon.
In addition, in order to support the connecting wall 22, a pair of support ribs 24 are formed between the spoke mounting wall 23 and the connecting wall 22 to interconnect the same such that a central compartment (a) and two lateral compartments (b), (c) are formed between the connecting wall 22 and the spoke mounting wall 23. The connecting wall 22 is formed with a plurality of through holes 25 to permit passage of spoke fasteners 28 that are to be retained at the spoke fastening holes 26. During manufacture, the bicycle wheel rim 2 is formed from an elongated metal strip by bending the strip and connecting two opposite ends of the strip. The resulting annular rim is then immersed in a liquid electrolyte to form an anodized coating on the surface thereof. Typically, the inner surfaces of the lateral compartments (b) and (c) are also formed with the anodized coating to prevent corrosion thereof. Therefore, the connecting wall 22 is further formed with fluid passage holes 27 on lateral end portions thereof to permit entry of the liquid electrolyte into the compartments (b) and (c), thereby complicating the manufacturing process. Moreover, before a tire (not shown) is mounted on the wheel rim 2, an inner lining (not shown) is provided on the connecting wall 22 to cover the through holes 25 so that the peripheries of the through holes 25 are prevented from damaging the tire. However, the lining is not designed to cover the fluid passage holes 27 formed on the lateral end portions of the connecting wall 22. Thus, the tire is in direct contact with the peripheries of the fluid passage holes 27, and is susceptible to damaging by the peripheries of the holes 27.